A membrane separation method of selectively permeating an objective gas utilizing the difference in gas permeability of a material is noticed as a method of concentrating a specific gas from a mixed gas. Because a mixed gas supplied often has high temperature and high pressure, an inorganic membrane such as a carbon membrane (for example, JP-A-2009-34614) or a ceramic membrane (for example, JP-A-3-267130) is the main stream as a membrane material. However, a polyimide membrane having high heat resistance and pressure resistance can be exemplified as a practical polymer membrane.
Particularly in recent years, the demand of a technology to separate, concentrate and recover carbon dioxide from natural gas and combustion gas is increased because of global warming. A gas to be treated in the technology undergoes modification of water vapor and aqueous gas shift and, therefore, contains carbon dioxide and hydrogen as main components. Gas permeability of the membranes described above is that hydrogen having small molecular size permeates faster than carbon dioxide and, therefore, it is impossible to selectively extract and concentrate carbon dioxide. Furthermore, because a gas that does not contain hydrogen such as natural gas contains water vapor, it is necessary to previously remove the water vapor by dehumidification before separation. Therefore, under the present situation, those membranes can be applied to only limited gases that do not contain hydrogen and water vapor.
In view of the above, a “physical absorption method” that absorbs in polyethylene glycol or the like under high pressure and a “chemical absorption method” that absorbs in an amine-based or ammonia-based solvent are adopted as a method of separating and concentrating carbon dioxide containing water vapor.
Furthermore, investigations of expanding a principle of a chemical absorption method to a membrane separation method are conducted. For example, liquid membranes in which a compound having affinity to carbon dioxide such as an organic liquid such as amine, or carbonate is covered with a polymer resin (for example, JP-A-2009-6260 and JP-T-2001-519711) are exemplified. In those liquid membranes, because a compound having high affinity involves facilitated transport of carbon dioxide, it is said that carbon dioxide selectively permeates a membrane as compared with hydrogen and helium.
In the physical absorption method and chemical absorption method described above, the cost of desorbing carbon dioxide absorbed in an affinity solvent is large, and absorption and desorption of carbon dioxide are repeated. Therefore, there is a problem that the life of the affinity solvent is short.
Furthermore, in the method using the liquid membrane described above, because the liquid membrane is constituted of a compound having an affinity to carbon dioxide and a polymer resin that does not have an affinity to carbon dioxide, a gas other than carbon dioxide such as hydrogen, permeates the polymer resin that does not have an affinity to carbon dioxide. As a result, selectivity of carbon dioxide is poor as compared to a chemical absorption method and the like.
That is, to stably separate and condensate carbon dioxide over a long period of time, it is effective that an organic liquid having an affinity to carbon dioxide is formed into a skin layer, and only carbon dioxide in a mixed gas permeates. However, there is a problem in the conventional technology that a gas to be removed such as hydrogen and helium (hereinafter referred to as a removal gas) permeates a polymer resin that fixes an organic liquid.
Furthermore, in the conventional technology, polyvinyl alcohol (PVA), an ethylene-polyvinyl alcohol copolymer (EVOH) and the like, that are easily compatible to an organic liquid and have small free volume have been used as a polymer resin.
However, only small free volume is not sufficient to suppress permeation of a removal gas such as hydrogen and, particularly, when a mixed gas supplied has high temperature, thermal motion becomes violent in an amorphous portion of a polymer and, as a result, permeability of the removal gas is increased. Furthermore, the rate of permeating a polymer resin is increased as a molecular size is decreased. For this reason, it is difficult to concentrate and recover carbon dioxide from a gas containing a molecule having small size such as hydrogen and helium, and carbon dioxide, and a new separation membrane has been desired.
Accordingly, it could be helpful to provide a carbon dioxide separation membrane that separates and concentrates carbon dioxide in high selectivity.